Cryptographic Algorithm and Key Usage Update to DKIM
draft-ietf-dcrup-dkim-usage-04

Document Type Active Internet-Draft (dcrup WG)
Last updated 2017-08-22 (latest revision 2017-08-21)
Replaces draft-kitterman-dcrup-dkim-usage
Stream IETF
Intended RFC status Proposed Standard
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Document shepherd Seth Blank
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Network Working Group                                       S. Kitterman
Internet-Draft                              Kitterman Technical Services
Updates: 6376 (if approved)                              August 21, 2017
Intended status: Standards Track
Expires: February 22, 2018

          Cryptographic Algorithm and Key Usage Update to DKIM
                     draft-ietf-dcrup-dkim-usage-04

Abstract

   The cryptographic algorithm and key size requirements included when
   DKIM was designed in the last decade are functionally obsolete and in
   need of immediate revision.  This document updates DKIM requirements
   to those minimaly suitable for operation with currently specified
   algorithms.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 22, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Discussion Venue  . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   4.  DKIM Signing and Verification Algorithms  . . . . . . . . . .   3
     4.1.  Key Sizes . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   4
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Discussion Venue

   RFC EDITOR: Please remove this section before publication.

   Discussion about this draft is directed to the dcrup@ietf.org [1]
   mailing list.

2.  Introduction

   DKIM [RFC6376] signs e-mail messages, by creating hashes of the
   message headers and content and signing the header hash with a
   digital signature.  Message recipients fetch the signature
   verification key from the DNS where it is stored in a TXT record.

   The defining documents specify a single signing algorithm, RSA
   [RFC8017], and recommends key sizes of 1024 to 2048 bits (but require
   verification of 512 bit keys).  As discussed in US-CERT VU#268267
   [VULNOTE], the operational community has recognized that shorter keys
   compromise the effectiveness of DKIM.  While 1024 bit signatures are
   common, stronger signatures are not.  Widely used DNS configuration
   software places a practical limit on key sizes, because the software
   only handles a single 256 octet string in a TXT record, and RSA keys
   significantly longer than 1024 bits don't fit in 256 octets.

   Due to the recognized weakness of the sha1 hash algorithm, see
   [RFC6194], and the wide availability of the sha256 hash algorithm (it
   has been a required part of DKIM [RFC6376] since it was originally
   standardized in 2007, the sha1 hash algorithm MUST NOT be used.  This

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